Monitoring the wastewater process is crucial for the effective management of water resource recovery facilities (WRRF).The measurement of manually sampled or mixed samples in the laboratory is the main method for compliance monitoring. However, process monitoring is increasingly being accomplished through online analytical instruments. Continuous monitoring provides key information for decision-making and reduces the burden on operators to sample and measure process parameters multiple times a day, enabling wastewater treatment plants to minimize chemical and energy inputs and avoid process anomalies. In the past decade, the selection and reliability of sensor technology have significantly improved. For example, sensors based on spectrophotometry can now be used to directly measure important parameters without the need for expensive reagents that need to be replenished frequently.

How to correctly select, debug, maintain, operate, and apply sensors based on spectrophotometry in wastewater treatment process monitoring? Many important substances in wastewater, including nitrate, nitrite, and organic carbon parameters, can be measured based on the absorption of ultraviolet and visible light (UV Vis). As an example of the UV visible light sensor used as part of the WTW IQ SensorNet process monitoring system under the Thermo Fisher brand, this article outlines its measurement principle, design, and operation.

Wastewater treatment monitoring

Monitoring wastewater treatment is crucial for verifying the effluent quality and evaluating treatment efficiency after each process stage. The municipal water resource recycling facilities mainly adopt activated sludge treatment technology. Activated sludge is a biological process that converts organic carbon into carbon dioxide (CO ₂). If denitrification is carried out, ammonia nitrogen is converted into nitrogen (N ₂) to remove dissolved and particulate pollutants.

BOD and COD are comprehensive parameters of many different substances, and measuring them separately requires a lot of analytical work. BOD and COD are indicators for measuring the amount of oxygen required for organic carbon oxidation in a sample. The measurement unit is mg O ₂/L. As for BOD, the reaction is catalyzed by bacteria. Apart from being limited to the laboratory due to the precise conditions required for long-term (5-day) cultivation, this method is highly sensitive to technology and may have precision issues. The COD method involves more intense chemical oxidation, where almost all organic carbon is oxidized. It is faster than BOD, but the implementation cost in online analyzers is high due to the need for sample collection, sample preparation equipment, and reagent supplementation. Total organic carbon (TOC) is used less frequently, but as a supplement or even replacement for BOD in process control, it is becoming increasingly popular. The TOC method is a direct measurement of organic carbon in the sample, with the measurement unit being mg C/L. TOC measurement is faster and simpler than BOD or COD, and does not require the use of mercury like COD methods. TOC measurement may also be more accurate, especially for samples with extremely low organic content, such as effluent from advanced water resource recovery facilities.

The sufficient removal of nitrogen from wastewater is becoming increasingly important. Nitrogen is a key conventional nutrient that supports the growth of organisms, including algae. In nutrient rich waters, the growth and death of algae accelerate, leading to hypoxia, which is a low oxygen condition that is not suitable for fish and other aquatic organisms. The treated wastewater is an important source of nutrients for certain watersheds. Therefore, monitoring and controlling all types of nitrogen is of great significance. The nitrate and nitrite in untreated wastewater can usually be ignored. Therefore, either of the two occurring in the treated wastewater is the result of biological nitrification, which converts ammonia in the wastewater into nitrate. Efficient nitrification can achieve maximum nitrate concentration. The sum of nitrite and nitrate (collectively referred to as NOx) is converted into nitrogen through another biological reaction called denitrification and removed from wastewater. If denitrification occurs in the aeration tank, it is a good thing for water resource recovery facilities because it will reduce the total nitrogen (TN) in the treated effluent and decrease the required energy intensity, as the reaction can be achieved without the need for oxygen input. However, if denitrification occurs in the sludge layer of the final sedimentation tank, it would be a bad thing. The nitrogen bubbles generated can cause solids to float up, interfere with the control of sludge return, and increase the total suspended solids (TSS) and BOD in the treated effluent when solids overflow the weir, which may result in violations of emission limits.

Limitations during nitrification or denitrification processes may lead to nitrite accumulation and even more compliance issues. Facilities with chlorine disinfection will immediately notice an increase in chlorine demand, as 1 mg/L of nitrite nitrogen will consume 5 mg/L of chlorine. Failure to keep up with the chlorine dosage can lead to insufficient disinfection and an increase in bacterial levels in the treated effluent. Other impacts may be more subtle, but there are still issues. Nitrite is highly toxic to aquatic organisms, even more so than nitrate. Nitrite nitrogen levels of 1 mg/L or higher may cause the entire effluent toxicity (WET) test to fail. The impact of elevated nitrite nitrogen levels is not limited to this. Strangely, bacteria that produce and consume nitrite (collectively known as nitrifying bacteria) are sensitive to nitrite, which can form a destructive feedback loop where a small disturbance can lead to a death spiral of nitrification and process abnormalities, resulting in long-term non-compliance with effluent ammonia and nitrogen limits. On the other hand, nitrite accumulation can be used to improve wastewater treatment efficiency. The understanding and development of short-range denitrification using nitrite have driven the development of innovative processes that intentionally shorten the denitrification process through nitrite, thereby reducing energy and chemical costs.

The form of nitrogen can be measured by many different methods, including ion chromatography, colorimetry, and potentiometry. Chromatography is very precise, but limited to laboratory applications. Colorimetric methods can also provide high-quality measurements, but require auxiliary equipment for sampling and sample preparation, as well as supplementary reagents.